Conditional display of data

Hi,

I had a requirement like below

     Col-1     Col-2

     10          aa

     20          aa

     30          aa

when ever string 'aa' occurs in one row , it has to replaced by 'bb'.

when ever string 'aa' occurs in two rows, in first row we have to display 'bb' and in second row 'bb1'

when ever string 'aa' occurs in 3 rows like above, in 1st row we have to display 'bb' and in 2nd row need to display 'bb1' and in 3rd row need to display 'bb2'

Here major problem is reading in how many rows it occurred.

Please provide your suggestions ASAP.

Thanks,

Subash